Today's fiber optic networks are highly intelligent, providing the efficient communication of information. The efficiency is largely enabled by the communication between nodes in fiber optic networks. For example, through inter-node communication, channel power can be monitored throughout the network, which allows for automated provisioning, such as channel power offsets at transmitter nodes to compensate for spectral ripple, gain at inline amplifiers nodes to compensate for span loss, and the like. For example, inter-node communication can achieved using a dedicated optical wavelength (1510 nm, 1625 nm, etc.), generally known as an optical supervisory channel (OSC) such as described in commonly assigned U.S. Pat. No. 6,765,659, issued Jul. 20, 2004 and entitled “OPTICAL SUPERVISORY CHANNEL APPARATUS AND METHOD FOR MEASURING OPTICAL PROPERTIES”, the contents of which are herein incorporated by reference.
Conventionally, a solution for detecting small span loss changes does not exist that satisfies all of the following criteria: first, the solution uses standard hardware typically found in a fiber optic network; second, the solution does not involve the use of additional hardware beyond that which is found in a typical fiber optic network; and third, the solution is capable of measuring small span loss changes (e.g., <1% power change, such as around 0.01 dB change in power).
Satisfying the first criterion is important from a feasibility standpoint. If the solution requires non-standard hardware, it may not be a feasible solution since the costs associated with using non-standard hardware generally outweigh the benefits. For example, small changes in span loss have been shown to be detectable with the use of multi-mode fiber (e.g., U.S. Pat. Nos. 5,003,623, 4,942,623, and 7,376,293). However, single-mode fiber typically dominates network infrastructures since its dispersive properties are more easily managed compared to multi-mode fiber, and these mechanisms are not adaptable to single-mode fiber.
The second criterion is important from a cost/complexity perspective. If the solution requires additional hardware, both the cost and complexity of the system increase. For example, small changes in span loss have been shown to be detectable with the use of a coherent light source and an interferometer (e.g., U.S. Pat. No. 5,194,847).
The first and second criteria can be satisfied in a fiber optic network that makes use of an OSC. In general, the OSC can be used to measure, among other properties, the span loss. It has been demonstrated that the OSC can be used to measure span loss changes by measuring the power of the OSC received at the output of a fiber span (OSC-Rx) and comparing it to the nominal power of the OSC transmitted at the input of the fiber span (OSC-Tx) (e.g., U.S. Pat. No. 6,891,607). While satisfying the first and second criteria, the technique does not satisfy the third criteria since it is not capable of measuring small span loss changes (e.g., <1% power change). This limitation is rooted in the assumption that the nominal OSC-Tx is constant. As a result, fluctuations in OSC-Tx are observed in OSC-Rx, rendering small power fluctuations in OSC-Tx indistinguishable from small span loss changes.